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. Cenozoic stable carbon (δ13C) and oxygen (δ18O)isotope ratios of deep-sea foraminiferal calcite co-vary with the 405 kyreccentricity cycle, suggesting a link between orbital forcing, the climatesystem, and the carbon cycle. Variations in δ18O are partlyforced by ice-volume changes that have mostly occurred since the Oligocene.The cyclic δ13C–δ18O co-variation is found inboth ice-free and glaciated climate states, however. Consequently, thereshould be a mechanism that forces the δ13C cyclesindependently of ice dynamics. In search of this mechanism, we simulate theresponse of several key components of the carbon cycle to orbital forcing inthe Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir model(LOSCAR). We force the model by changing the burial of organic carbon in theocean with various astronomical solutions and noise and study the responseof the main carbon cycle tracers. Consistent with previous work, thesimulations reveal that low-frequency oscillations in the forcing arepreferentially amplified relative to higher frequencies. However, whileoceanic δ13C mainly varies with a 405 kyr period in themodel, the dynamics of dissolved inorganic carbon in the oceans and ofatmospheric CO2 are dominated by the 2.4 Myr cycle of eccentricity.This implies that the total ocean and atmosphere carbon inventory is stronglyinfluenced by carbon cycle variability that exceeds the timescale of the405 kyr period (such as silicate weathering). To test the applicability ofthe model results, we assemble a long (∼22 Myr) δ13C andδ18O composite record spanning the Eocene to Miocene(34–12 Ma) and perform spectral analysis to assess the presence of the2.4 Myr cycle. We find that, while the 2.4 Myr cycle appears to beovershadowed by long-term changes in the composite record, it is present asan amplitude modulator of the 405 and 100 kyr eccentricity cycles.

The 405 kyr and 2.4 Myr eccentricity components in Cenozoic carbon isotope records